auth-app.js: Throw a descriptive error when permissions is not set

Hello,

I ran into an issue where when I did not set a permissions object, even an empty one, in the the auth options I would see the following error:

Cannot convert undefined or null to object

Upon debugging it seems as if it comes from here:

https://github.com/octokit/auth-app.js/blob/0b50bb41fb850bb3398f0449bd3a4438ff04acff/src/cache.ts#L72

If no permissions are passed in, the Object.keys throws the Cannot convert undefined or null object.

It was not clear from the documentation that the permissions object is required. Examples in the documentation seem to leave that out.

Small Reproducer

const auth = createAppAuth({
  id: 123,
  privateKey: `-----BEGIN RSA PRIVATE KEY-----...`,
  installationId: 456,
});

const installationAuthentication = await auth({
  type: 'installation',
  // permissions: {}, // This is what I added to make it work
});